1. Field of the invention
The invention is in the field of optical transmission using wavelength-division multiplexes. It concerns in particular a method for selectively adding and/or dropping one or more channels of a multiplex.
Such operations are useful in routing and switching functions. For example, they can be used to modify the information on one or more channels or to change the wavelength of a channel carrying information. An add and drop system performing these operations is usually referred to as an optical add and drop multiplexer (OADM).
2. Description of the Prior Art
The method of the present invention includes the following operations disclosed in patent document EP-A-0 838 918 published on Apr. 29, 1998, although the terminology used in the following description of these operations differs from that used in the above document:
information to be transmitted being carried by a plurality of channels, some of said channels are assembled to form an upstream multiplex, the channels of that multiplex being distributed in the wavelength spectrum, the channels of the upstream multiplex are separated in a selection module into at least one dropped group and one transit group,
the dropped group is diverted to a lateral circuit,
at least during reinsertion periods, at least a part of the dropped group is diverted to an optical amplifier, this amplifier constituting a lateral amplifier, and
the transit group is assembled with said part of the dropped group to form a downstream multiplex.
The reinsertion periods occur when the dropped group has not been modified.
In practise, the lateral circuit causes an unwanted loss of power from the dropped group. In any transmission system it is desirable for each multiplex to be balanced, i.e. for all the channels of the multiplex to be at substantially the same power. In the usual case where the upstream multiplex is balanced, a general problem therefore arises of balancing the downstream multiplex, and a more particular problem is to achieve this balancing during said reinsertion periods despite said losses in the lateral circuit.
The following solutions to this more particular problem might be considered.
In a first solution, the selection module offers the transit channel group a circuit that is not used by the drop channel group. This circuit then includes an attenuator for compensating the losses in the lateral circuit, possibly with the aid of the lateral amplifier. This first solution has two drawbacks, one being that the power of the channels would be lower in the downstream multiplex than in the upstream multiplex, which would require an amplifier at the output of the selection module, and the other being that this module would be more complex and costly than the usual modules.
In a second solution, the gain of the lateral amplifier is varied in order to compensate for the losses in the lateral circuit. This second solution is not described in patent document EP-A 0 838 918, which indicates for this amplifier only a switching function, and the multiplication factor defining the gain of an amplifier implementing this function is typically 0 to open an optical circuit including the amplifier or 1 to transmit the optical waves without changing their power. It would have the drawback that the initial setting of the gain of the amplifier would be difficult and that the compensation of losses might then be disrupted by modifications of this gain caused by variations in the power of the upstream multiplex and/or by drift of the specifications of the amplifier itself or by the units pumping it.
A third solution is described in an article by F. Shehadeh, R. S. Vodhanel, M. Krain, C. Gibbons, R. E. Wagner and M. Ali: xe2x80x9cGain-Equalized, Eight-Wavelength WDM Optical Add-drop Multiplexer with an 8-dB Dynamic Rangexe2x80x9d, IEEE Photonics Technology Letters, vol. 7, Nxc2x0 Sep. 9, 1995, pp 1075-1077: the channels of the outgoing multiplex of the add and drop system are separated from each other in a demultiplexer, after which the power of each of the channels is adjusted to a common value, after which the channels are superposed to constitute a balanced downstream multiplex. This solution has the drawback of being costly.
A particular object of the present invention is to find a method of balancing the outgoing multiplex of an add and drop system permanently and at low cost when a multiplex received by the system is balanced and the system extracts some of the channels from the received multiplex and then reinserts the same channels into the outgoing multiplex. A more general object of the present invention is to prevent a system of the above kind modifying the ratios of the respective powers of the channels of a multiplex when some channels are dropped and then added in this way.
It exploits two facts to achieve the above objects. One of these facts is that prior art optical amplifiers that can be used for the lateral amplifier are progressive saturation amplifiers, i.e. have a gain that decreases with the total power of the optical waves received by the amplifiers to be amplified therein. The other fact is that, in the low-cost modules constituting the selection module of prior art add and drop systems, optical waves could be guided in a channel different to those used in the prior art systems.
The invention consists in a method of dropping and adding channels for use in wavelength-division multiplex optical transmission, the method including the following operations:
information to be transmitted being carried by a plurality of channels, some of the channels being assembled to form an upstream multiplex, and the channels of the multiplex being distributed in the wavelength spectrum, the channels of the upstream multiplex are separated in a selection module into at least one dropped group and one transit group,
the dropped group is diverted to a lateral circuit,
at least during reinsertion periods, at least a part of the dropped group is diverted to an amplifier system having a progressive saturation gain, and
the transit group is assembled with the part of the dropped group to form a downstream multiplex,
in which method the gain of the amplifier system is made sufficient to give rise to an optical wave in the lateral circuit and the amplifier system at the same time as the part of the dropped group, the selection module being adapted to enable the wave to circulate in a closed loop further including the module and the wave constituting a loop wave, and a preferred wavelength for the loop wave is outside a spectrum occupied by the channels.
The invention also consists in an add and drop system adapted to receive an upstream multiplex made up of channels divided in wavelength in a transmission spectrum, the system including:
a selection module separating the channels of the upstream multiplex into at least one dropped group and one transit group and forming a downstream multiplex by assembling the transit group with an added group received by the module and consisting of at least one channel at a different place in the spectrum of wavelengths than the transit group, and
a lateral unit adapted to form a lateral circuit transmitting at least a part of the dropped group to the selection module to constitute the added group so that the added group then constitutes a reinserted group, the lateral circuit including an amplifier system having a progressive saturation gain,
in which system the gain of the amplifier system has values sufficient to give rise to an optical wave in the lateral circuit and the amplifier system at the same time as the reinserted group, the selection module enabling the wave to circulate in a closed loop also including the module, the wave constituting a loop wave, the loop constituting a gain adjustment loop, and the system further including a spectrum locking element in the gain adjustment loop and favoring a wavelength external to the transmission spectrum so as to impose on the loop wave a wavelength also external to the spectrum.
Various embodiments of the invention are described hereinafter with the aid of the accompanying drawings. If a component is shown in more than one figure, it is always designated therein by the same reference letters and/or numerals.